Photoelectric imaging devices include line-focus systems that scan an image provided on a medium by sequentially focusing narrow scan-line portions of the image onto a sensor array by moving an optical imaging head assembly relative to the medium. Such line-focus systems are commonly referred to as optical scanners. As illustrated in FIG. 1, a portion of a conventional optical scanner 10 for scanning a medium 11 that has a longitudinal medium axis and a transverse medium axis includes a housing 12, an optical imaging head assembly 14, and a platen 16. The housing includes a longitudinal axis 18 and a transverse axis 20. The longitudinal axis of the housing is substantially parallel to the longitudinal medium axis, and the transverse axis of the housing is substantially parallel to the transverse medium axis. These relationships are maintained as the medium is scanned. The optical imaging head assembly includes an assembly axis 22. In the conventional optical scanner, the transverse axis is substantially perpendicular to the longitudinal axis of the housing and the optical imaging head assembly is oriented such that the assembly axis is also substantially perpendicular to the longitudinal axis of the housing. The optical imaging head assembly is mounted in the housing for movement along the longitudinal axis of the housing in the directions indicated by double arrow 24. The platen is mounted on the housing over the optical imaging head assembly such that the optical imaging head assembly moves underneath the platen along the longitudinal axis of the housing to scan an image of the medium placed face down on the platen. Thus, when the optical imaging head assembly moves along the longitudinal axis of the housing, the assembly axis of the optical imaging head assembly remains parallel to the transverse axis of the housing and the transverse medium axis.
In the conventional optical scanner, the optical imaging head assembly, commonly referred to as a scanner bar or a scanning head, typically includes a light source and a sensor array, such as a charged coupled device (CCD) or a contact image sensor (CIS), which includes light receptors that detect variations in light intensity and frequency by building up an electrical charge in response to exposure to light for a preset period of time. As such, the light source illuminates the surface of the medium and the sensor array converts reflected or transmitted light from the surface into electrical signals. Each element or cell of the sensor array corresponds to a small area, commonly referred to as a picture element or pixel, and produces a data signal that is representative of the intensity of light from the area. As such, each cell has a portion of a scan-line image impinged thereon as the optical imaging head sweeps across the image. The electrical signals can then be stored in a file, manipulated by programs, and/or used for reproduction of the image.
Halftoning is a technique used to create images with varying shades or levels of gray or other colors. More specifically, with halftoning, patterns of closely spaced individual dots of black or an appropriate color, such as cyan, yellow, or magenta, are formed to create an image. Thus, by using halftoning, the illusion of more grays or colors other than those within a color gamut of a device, such a display or printer, is created. The varying shades or levels of gray or other colors are achieved by varying the size and/or spacing of the individual dots. The dots of each respective color in a color halftone image are arranged in rows of dots such that the rows of dots of each respective color are typically inclined at different angles with respect to the vertical. Thus, spacing of the rows establishes a halftone resolution of the image and an angle of each of the rows establishes a halftone angle of the image.
Unfortunately, scanning a halftone image with the conventional optical scanner may create image artifacts or defects that degrade image quality and detract from the appearance and usability of the scanned image. For example, at certain scanning resolutions, the image pixels and halftone dots may interfere, thereby creating moiré patterns represented by bands or blotches across the scanned image. This happens when a physical resolution or frequency of the optical imaging head assembly of the conventional optical scanner is too close to the halftone resolution of the halftone image. As a result, the conventional optical scanner thereby picks up alternating light and dark parts of the halftone image resulting in bands and blotches across the scanned halftone image. Attempts to remove such artifacts have included high resolution scanning and the use of computationally intensive algorithms, which tend to be relatively slow or require a powerful computer. Also, simple averaging algorithms that are used to filter the image data tend to be inaccurate, which may create other kinds of image distortions.
Accordingly, a need exists for preventing artifacts from appearing in scanned half tone images without having to rely on high resolution scanning or computationally intensive algorithms to remove artifacts from already scanned halftone images.